Carbon Nanotubes for Solar Energy (2/2)

Carbon nanotubes in photovoltaics

Don’t you love it when the sun wakes you up early in the morning? The sun is kinda like this:

Carbon nanotubes in photovoltaics

Organic photovoltaic devices (OPVs) are created from thin films of organic semiconductors like polymers and are typically just 100 nm thick.

Organic Solar Cells!!
Structure of a bilayer organic photovoltaic device
  • Due to the concentration gradient, the excitons diffuse to the donor/acceptor interface (Exciton Diffusion) and separate into free holes (positive charge carriers) and electrons (negative charge carriers) (Charge Separation).
  • A photovoltaic is generated when the holes and electrons move to the corresponding electrodes by following either donor or acceptor phase (Charge Extraction).
(D = donor, A = acceptor)

Carbon nanotube composites in the photoactive layer

But first, let’s learn about PN Junction.

The PN junction is a junction formed together by P-type semiconductor and N-type semiconductor material. The junction plays an important role in the development of the diode which is the building block of most of the semiconductor devices like transistors and solar cells.

  • The region where both N-type and P-type material are joined together is called junction (or boundary of the semiconductor).
  • The majority charge carriers in the P region are holes, and major charge carriers in N-region are electrons.
  • Key Idea: When the junction is formed, the holes present in the P region are diffused into the N-region, leaving behind the negative charge in the P region which then recombines with electrons, resulting negatively charged ions in the P-region.
  • Key Idea: In this same way, electrons present at the N-region diffuse into the P-region leaving behind the positive charge in the N-region and recombine with the holes, creating positively charged ions in the N-region.
  • As the junction is formed each region of silicon crystal becomes depleted from major charge carriers around the junction. This region is known as depletion or space charge region. The width of this region highly affects the current flowing between the junction.

Now, what is Conjugation?

In organic chem, Conjugation is used to describe the situation that occurs when π systems (e.g. double bonds) are “linked together”.

  • An “extended” π (pi) system exists over a longer series of atoms (e.g. C=C-C=C or C=C-C=O etc.).
  • An extended π (pi) system results in an extension of the chemical reactivity.
  • If a position in the chain does not provide a “p” orbital, then the conjugation is interrupted (broken).

Conjugated Polymers

Conjugated polymers have unique material properties because of their ability to virtually create new materials by simply tuning the molecular structure.

Single Wall carbon nanotubes (SWCNTs) for light

The photovoltaic effect can be achieved in ideal single wall carbon nanotube (SWNT) diodes. The PN-junction diode acts like solid state one-way electrical valve that only allows electrical current to flow through themselves in one direction only. The advantage of this is that diodes can be used to block the flow of electric current from other parts of an electrical solar circuit.

Carbon Nanotube -> Diode

Carbon nanotubes as a transparent electrode

Indium tin oxide (ITO) is currently the most popular material used for the transparent electrodes in OPV devices but it has a TON of errors like not being very compatible with polymeric substrates because of its high deposition temperature. Traditional ITO also has unfavorable mechanical properties like being relatively fragile.

Nanotube arrays based flexible perovskite solar cells with transparent carbon nanotube electrode

CNTs in dye-sensitized solar cells

Due to the simple fabrication process, low production cost, and high efficiency, there is significant interest in dye-sensitized solar cells (DSSCs). It is based on a semiconductor formed between a photo-sensitized anode and an electrolyte, a photoelectrochemical system.

Dye-sensitized solar cells

Future Applications

These are just some of the MANY ways in which Carbon Nanotubes are being leveraged for Solar Energy. Things like Carbon Nanotube Hybrids for Renewable Energy or even recently NovaSolix is working on a carbon nanotube-based solar module which has the potential to reach 90% efficiency!!!

Key Takeaway from this article

Hi. I’m Alishba.

I’m so excited to be working in the space of clean energy + sustainability using Nanotechnology, for the next few months. I’d love to connect with you and chat more about the tech + research I’m doing. Feel free to reach out!

I’m a developer & innovator who enjoys building products and researching ways we can use AI, Blockchain & robotics to solve problems in healthcare and energy!